DDGS from most modern U. S. fuel ethanol plants typically contains about 30% protein, 10% fat, at least 40% neutral detergent fiber, and up to 12% starch (Rosentrater and Muthukumarappan, 2006). Composition, however, can vary between plants and even within a single plant over time, due to a number of factors. For example, Table 1 summarizes composition of DDGS samples collected from five ethanol plants in South Dakota. On a dry basis, crude protein levels ranged from 28.3 to 31.8%; crude lipid varied between 9.4 and 11.0%; ash ranged from 4.1 to 13.3%. In terms of within-plant variability, the crude protein, crude lipid, and starch content all exhibited relatively low variation, whereas neutral detergent fiber (NDF), acid detergent fiber (ADF), and ash all had substantially higher variability.

Furthermore, DDGS from 49 plants from 12 states were analyzed for proximate composition (Table 2) and amino acid profiles (Table 3) (UMN, 2011). Dry matter content varied from 86.2% to 92.4%, while protein varied from 27.3% to 33%. Crude fat content displayed even higher variability, and ranged from 3.5% to 13.5%; crude fiber ranged from 5.37% to 10.58%; and ash content varied from 2.97% to 9.84%. On average, geographic trends were not readily apparent for any of the nutrient components. In terms of amino acids, lysine ranged from 0.61% to 1.19%, but again, no geographic trends were apparent.

Some plants are beginning to implement various fractionation processes (either pre­fermentation or post-fermentation) in order to produce multiple product streams (RFA, 2009a). These new processes can lead to additional differences in DDGS nutrient levels. For example, various techniques for dry fractionation and wet fractionation have been developed to concentrate protein, fiber, and oil components from the endosperm (which contains the starch). This allows a highly-concentrated starch substrate to be introduced to the fermentation process, and it allows the other components to be used for human food applications. Singh and Johnston (2009) have provided an extensive discussion regarding various pre-fermentation fractionation approaches. On the other hand, post-fermentation fractionation techniques have also been examined. For example, Srinivasan et al. (2005) used a combination of (air classification and sieving to separate fiber particles from DDGS. Processes have also been developed to remove corn oil from thin stillage and CDS; although the resulting corn oil fractions cannot be used as food-grade oil, they can readily be converted into biodiesel. All of these approaches, if implemented commercially, will alter the composition of the resulting DDGS.

The U. S. ethanol industry’s primary market for distillers grains has historically been as a commodity livestock feed. Most often this has been in the form of DDGS, and to a lesser degree in the form of DWG; the other coproducts are sold in much lower quantities than either DDGS or DWG and some are not always produced either). Feeding ethanol coproducts to animals is a practical method of utilizing these materials because they contain high nutrient levels, and they are digestible (to varying degrees) by most livestock. And, use of DDGS in animal feeds (instead of corn grain) helps to offset the corn which has been

DDGS use in livestock diets has continued to increase over the years. Predictions of peak potential for DDGS use in domestic U. S. beef, dairy, swine, and poultry markets have estimated that between 40 and 60 million t could be used in the U. S. each year, depending upon inclusion rates for each species (Staff, 2005; Cooper, 2006; U. S. Grains Council, 2007). Globally, the need for protein-based animal feeds continues to grow. Of the 23 million t of DDGS produced in 2008 (RFA, 2009b), 4.5 million t were exported to international markets (FAS, 2009); this accounted for nearly 20% of the U. S. DDGS production that year (Figure 6). And the potential for global exports is projected to increase for the foreseeable future (U. S. Grains Council, 2007).

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Not only are coproducts important to the livestock industry as feed ingredients, but they are also essential to the sustainability of the fuel ethanol industry itself. In fact, the sale of distillers grains (all types — dry and wet) contributes substantially to the economic viability of each ethanol plant (sales can generally contribute between 10 and 20% of a plant’s total revenue stream (Figure 7), but at times it can be as high as 40%), depending upon the market conditions for corn, ethanol, and distillers grains. This is the reason why these process residues are referred to as "coproducts", instead of "byproducts" or "waste products"; they truly are products in their own right along with the fuel.

So the sales price of DDGS is important to ethanol manufacturers and livestock producers alike. Over the last three decades, the price for DDGS has ranged from approximately $50.71/1 up to $209.44/1 (Figure 8). DDGS and corn prices have historically paralleled each other very closely (Figure 9). This relationship has been quite strong over the last several

years. This is not surprising, as DDGS is most often used to replace corn in livestock diet formulations. DDGS has increasingly been used as a replacement for soybean meal as well, primarily as a source of protein. Even so, DDGS has historically been sold at a discounted price vis-a-vis both corn and soybean meal. This has been true on a volumetric unit basis, as well as per unit protein basis (Figure 9).